Liquid crystal displays are used as computer displays, control panels of home appliances, cellular phone displays, and the like. There is an increasing demand for reducing the power consumption, weight, and thickness of the liquid crystal display. The liquid crystal display typically employs a transmissive liquid crystal panel.
The liquid crystal display of this type employs a surface light source as a backlight to illuminate a liquid crystal panel from the back. The surface light source has a light guide to emit areal light based on light from a light source and an optical film to control the direction of outgoing light of the light guide so that as much light as possible is emitted toward the front of an observer who watches the liquid crystal display.
FIG. 1 shows a configuration of a liquid crystal display according to a related art.
The liquid crystal display has a light source 111 such as a light emitting diode for emitting white light, a light guide to guide the light from the light source 111 and emit areal light, an optical film 110 such as a prism sheet to bend light 114 obliquely emitted from the light guide 112 into a direction perpendicular to an exit surface, a diffuser 132, and a transmissive liquid crystal panel 130 to display images. From the liquid crystal display toward an observer, light 134 illuminated the liquid crystal panel 130 is emitted.
The diffuser 132 is arranged to suppress moire fringes caused by periodical prism or hologram structures of the optical film 110 and periodical pixel intervals in the liquid crystal panel 130, subdue Newton rings occurring between the liquid crystal panel 130 and the optical film 110, and reduce a chromatic dispersion of light 116 emitted from the optical film 110. The diffuser 132, however, adds interfaces and decreases brightness due to Fresnel reflection by the interfaces. To cope with this, there has been proposed an idea to form the diffuser 132 on the exit surface of the optical film 110. Japanese Unexamined Patent Application Publications No. 9-281310 and No. 9-281311 include microscopic beads or rods in an exit surface of an optical film to roughen the exit surface with irregularities and provide the exit surface with a light diffusing ability. This technique may suppress moire fringes and Newton rings but is unable to control the range of diffusion angles of the microscopic beads or rods. As a result, light may substantially uniformly diffuse into every direction, to drastically deteriorate front brightness.
In the liquid crystal display, the light source 111, light guide 112, optical film 110, and diffuser 132 form a surface light source for supplying areal light to the liquid crystal panel 130.
FIG. 2 shows a relationship between incident light 114 and outgoing light 116 on the optical film 110.
The incident light 114 to the optical film 110 forms an incident angle θi and the outgoing light 116 from the optical film 110 forms an output angle θo. The incident angle θi and output angle θo are angles made by the incident light 114 and outgoing light 116 with respect to normals of the incident and exit surfaces of the optical film 110, respectively.
The incident angle θi with respect to the optical film 110 is dependent on the design of the light guide 112 and is in the range of 20° to 80°. A role of the optical film 110 is to efficiently bend light obliquely made incident from the light guide 112 to the incident surface into a direction perpendicular to the exit surface, i.e., a direction in which the output angle θo is 0°. For this, the material and shape of the optical film 110 must be designed to reduce Fresnel reflection, i.e., interfacial reflection between an air layer and the optical film 110 and maximize light that advances in the 0° direction. If the outgoing light 116 has an angular distribution, the optical film 110 is provided with a light bending characteristic so as to maintain brightness in a perpendicular direction even if the incident angle θi slightly varies. This may increase brightness in a front direction compared with providing a fixed light bending angle. The light source emits white light, and therefore, it is necessary to reduce wavelength-dependent dispersion and suppress unevenness and blurs in images displayed on the liquid crystal panel 130.
The optical film 110 and light guide 112 employ a refraction law such as the Snell's law to bend outgoing light in a geometrical-optical manner. In this case, the optical film 110 may be made of a prism sheet having an incident surface provided with prism grooves and ridges. The prism sheet of this type is structurally simple and is easy to manufacture.
In place of the conventional refractive prism sheet, a hologram optical film may be used. The hologram optical film utilizes a diffraction phenomenon based on the wave characteristic of light. Employing the hologram realizes a function of bending light as well as a function of condensing light. A method of designing an optical element with a hologram is disclosed in, for example, Victor Soifer, Victor Kotlyar, and Leonid Doskolovich, “Iterative Methods for Diffractive Optical Elements Computation,” Taylor Francis (1997).
The hologram has been considered improper for bending white light because it causes spectral dispersion and high-order diffraction.
It is known that obliquely emitting light from a light guide provides the emitted light with directivity and that bending the directive light in a perpendicular direction improves brightness. (For example, refer to Japanese Patent Publication No. 2739730.)
An optical film employing any one of the prism sheet and hologram is required to further increase the brightness of outgoing light oriented in a direction controlled by the optical film.
As mentioned above, perpendicularly bending directive light obliquely emitted from a light guide improves the brightness of the light. It is difficult, however, to align output angles over the surface of the light guide. Then, brightness unevenness easily occurs. There is an idea to insert a diffuser capable of maintaining brightness and eliminating brightness unevenness between the light guide and the optical film. To achieve this, the diffuser must have a limited range of diffusion angles. The diffuser having a limited range of diffusion angles, however, hardly secures a required diffusion characteristic to control a brightness distribution if light is obliquely made incident thereto from the light guide.